linux-kernel - Re: [PATCH v4 4/4] seccomp: add support for passing fds via USER

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Message-ID: <20180622005134.GJ3992@cisco>
Date:   Thu, 21 Jun 2018 18:51:34 -0600
From:   Tycho Andersen <tycho@...ho.ws>
To:     Jann Horn <jannh@...gle.com>
Cc:     Kees Cook <keescook@...omium.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        containers@...ts.linux-foundation.org,
        Linux API <linux-api@...r.kernel.org>,
        Andy Lutomirski <luto@...capital.net>,
        Oleg Nesterov <oleg@...hat.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Tyler Hicks <tyhicks@...onical.com>,
        suda.akihiro@....ntt.co.jp, "Tobin C. Harding" <me@...in.cc>
Subject: Re: [PATCH v4 4/4] seccomp: add support for passing fds via
 USER_NOTIF

On Fri, Jun 22, 2018 at 01:34:18AM +0200, Jann Horn wrote:
> On Fri, Jun 22, 2018 at 12:05 AM Tycho Andersen <tycho@...ho.ws> wrote:
> >
> > The idea here is that the userspace handler should be able to pass an fd
> > back to the trapped task, for example so it can be returned from socket().
> [...]
> > +Userspace can also return file descriptors. For example, one may decide to
> > +intercept ``socket()`` syscalls, and return some file descriptor from those
> > +based on some policy. To return a file descriptor, the ``return_fd`` member
> > +should be non-zero, the ``fd`` argument should be the fd in the listener's
> > +table to send to the tracee (similar to how ``SCM_RIGHTS`` works), and
> > +``fd_flags`` should be the flags that the fd in the tracee's table is opened
> > +with (e.g. ``O_EXCL`` or similar).
> 
> fd_flags only contains file descriptor flags (meaning only O_CLOEXEC).
> O_EXCL is a file creation flag, so setting it here wouldn't make sense.
> Setting file status flags like O_APPEND does make sense, but those are
> stored in the `struct file` and don't need to be passed separately;
> the caller can e.g. set them via fcntl(fd, F_SETFD, flags) or on
> open().
> (The fcntl.2 manpage explains these.)

Ugh, yes, O_CLOEXEC is what I meant. Thanks, I'll clarify.

Tycho